Silver, silver chloride electrodes are widely used in electrochemical and biomedical applications. For instance, in EKG application, Ag/AgCl electrodes are used to detect very weak electrical responses from human hearts, and electrodes with high conductivity and low electrode polarization are desirable to achieve low noise and high signal sensitivity. Another application involves the use of Ag/AgCl electrodes in electrochemical applications, such as electrophoresis where a continuous electrical current is applied to facilitate the transport of charged particles. In such application Ag/AgCl electrodes allow the delivery of a continuous current at a low and steady voltage. Because of the ability of Ag/AgCl electrodes to maintain a constant and low standard electrode potential. Ag/AgCl is widely used as a reference electrode. Still another application is use as a biosensor. A biosensor consists of a biological component, typically in the form of a polymer membrane and a transducer that is structurally integrated to a biological component. The transducer converts the biological signal to a form of an electrical signal that can be measured directly or amplified further to produce analytical results. An Ag/AgCl electrode functions as a counter electrode vs an enzyme/platinum working electrode, when a stable electrode potential is important. All applications herein are based on the electrochemical characteristics of a Ag/AgCl electrode, namely, (a) low half-cell potential vs standard hydrogen electrode, (b) minimum electrode polarization, (c) stable electrode potential under a low current bias.
Conventional Ag/AgCl electrodes are manufactured in several ways, namely, (a) electrochemically treating silver foil to form a thin surface layer of silver chloride on silver foil, (b) forming Ag/AgCl disk electrodes by compaction of silver and silver chloride particles, and (c) coating of a silver/silver chloride polymer composition on a dielectric substrate. In the utilization of EKG electrodes or medical electrodes, the Ag/AgCl electrodes are further coated with a saline water-containing hydrogel which serves as an ionic conducting media and a skin adhesive for attachment to human skin.
Of the three methods described, the use of silver/silver chloride polymeric inks printed on plastic film substrates is particularly attractive from cost and performance standpoints. With polymeric inks, printing can be carried out by flexographic, gravure or screen printing processes to produce thin Ag/AgCl polymer coatings of 0.2-0.3 mil on plastic films, such as polyester, polycarbonate, polyvinyl chloride and the like. The coated film can then be stamped out into small pieces to make low-cost, disposable electrodes for EKG and other medical electrode applications.
Silver, silver chloride polymer compositions disclosed in the prior art are typically prepared by dispersing silver and silver chloride particles in solvent based polymer solutions. U.S. Pat. No.5,051,208 discloses screen printable Ag/AgCl paste compositions with polyester or phenoxy resins as the polymeric binders. U.S. Pat. 5,207,950 discloses polymeric paste compositions with chloride silver particles. The Ag/AgCl polymer compositions disclosed by the art teach organic solvents as the printing vehicle. With increasingly stringent regulations aimed at reducing air emission of organic solvents from coating industries, there is a need for ink products with low volatile organic compounds (VOC). Water based Ag/AgCl ink is an attractive alternative to meet such a need. Further, needs exist to reduce cost of the disposable biomedical electrodes through more efficient usage of silver and silver chloride in the printing inks while improving the required electrochemical characteristics of these electrodes. It is the objective of the present invention to provide conductive polymeric coating compositions for biomedical and electrochemical electrodes that surpass emission standards and remedy the above mentioned shortcomings.